The present invention relates to a magnetic disk drive, and more particularly to a magnetic disk drive provided with a retract circuit.
Magnetic disk drives are provided with a magnetic disk and a magnetic head to effect read/write operations. In a normal operating condition, the magnetic disk rotates at a high speed and the magnetic head is floated slightly from the magnetic disk surface by air flow. This prevents that scratches occur on the recording surface of the magnetic disk to damage the data recorded thereon.
As stated above, since the magnetic disk has an arrangement such that the magnetic head is caused to be floated by air flow when the magnetic disk normally rotates at a high speed, there is no possibility that scratches occur on the data recording zone on the magnetic disc. However, when the rotation speed of the magnetic disk becomes slow due to the fact that an input power supply or the magnetic disk drive itself is in abnormal condition, or for other reasons, the magnetic head is not floated to become in contact with the data recording zone, giving rise to the possibility that scratches occur on the data recording zone. The magnetic head is in contact with a predetermined zone except for the data recording zone on the magnetic disk before the rotation of the magnetic disk is initiated, or when the magnetic disk is stopped. The predetermined zone is called an "landing zone".
In a typical magnetic disk drive, when the magnetic disk rotates at a high speed and the magnetic disk drive normally operates, the magnetic head is positioned above the data recording zone. In contrast, before the rotation of the magnetic disk is initiated, when the magnetic disk is stopped, and when the drive is in abnormal condition, the magnetic head is caused to be transferred to the landing zone except for the data recording zone on the magnetic disk. An electric circuit for transferring the magnetic head to the stop area in this manner is usually called a "retract circuit".
FIG. 3 is a block diagram illustrating an example of a retract circuit employed in the conventional magnetic disk drive. This retract circuit has an arrangement such that a voice coil la provided in a voice coil motor 1 for driving a carriage on which the magnetic head is mounted is switched to either a power supply 9 or a motor drive circuit 3 through contacts 2a and 2b provided in a relay 2 and is connected thereto.
The motor drive circuit 3 is connected to an analog switch 5 through a motor control circuit 4. This analog switch 5 responds to a retract signal 7 from a microcomputer 6 functioning as a control circuit for controlling the entirety of the retract circuit to deliver a constant current determined by a resistor 8 and a voltage V to the motor control circuit 4. In addition, for controlling the motor control circuit 4, a control signal is inputted from the microcomputer 6.
Before the circuit is powered, the contacts 2a of the relay 2 are closed in response to a motor off command of the motor signal. Simultaneously with the energization of the circuit, the relay 2 is switched so that the contacts 2b are closed. Thus, the retract circuit is placed in controllable state by the retract signal 7 from the microcomputer 6.
In such a condition, when the abnormality that the number of revolutions of the magnetic disc is decreased is detected by hardware or the magnetic disk drive is in abnormal state, e.g., a servo circuit for positioning a magnetic head is abnormal, the retract signal 7 is delivered from the microcomputer 6. Upon receiving this retract signal 7, the analog switch 5 delivers a constant current determined by the resistance value of the resistor 8 and the voltage V to the motor control circuit 4.
A force corresponding to the constant current thus delivered is transmitted to the voice coil motor 1 through the motor drive circuit 3 and the magnetic head is driven in the inner peripheral direction of the magnetic disk to move the magnetic head to the landing zone provided in advance.
In this instance, it is desirable that this drive force is always an optimum value. As apparent from FIG. 3, since the value of a current delivered to the motor control circuit 4 is determined by the resistance value of the single resistor 8, when the retract signal 7 is delivered, a certain constant current is delivered to the motor control circuit 4 in any case, so that the drive force is always maintained constant. Accordingly, it happens that this drive force is too weak or too strong dependent upon the magnetic head position or the direction in which the drive is installed.
FIG. 4 is a plan view illustrating the surface of the magnetic disk 10. The data recording zone 13 having data tracks for recording data is provided outside the landing zone 11. The innermost data track of the data recording zone 13 is called an "innermost cylinder 12" and the outermost data track of the data recording zone 13 is called an "outermost cylinder 14".
In such a magnetic disk drive, where a comparison between a force necessary for moving it from the innermost cylinder 12 to the landing zone 11 and a force necessary for moving it from the outermost cylinder 14 to the landing zone 11 is made, these forces are ordinarily different from each other.
However, when the conventional retract circuit is employed, the same drive force is always exerted on the magnetic head. For this reason, there is the possibility that if this drive force is too strong, the carriage forcibly collides with a stopper for stopping the magnetic head from moving it inside the landing zone 11 to damage the magnetic head, and if this drive force is too weak, the magnetic head cannot be reached to the landing zone 11.
FIGS. 5(A), 5(B) and 5(C) are perspective views illustrating examples of the installation of the magnetic disk drive 17. As seen from these figures, three different relative positional relationships between the magnetic disk 10 and the carriage 16 on which the magnetic head 15 is mounted are conceivable.
In the case where the magnetic disk 10 is installed in parallel with ground as shown in FIG. 5(A), even if the carriage 16 is always driven by a constant drive force, there is no problem. However, in the case where the magnetic disk 10 is installed perpendicular to ground as shown in FIG. 5(B) or FIG. 5(C), if the drive force of the carriage 16 is constant, there occurs a big problem. Namely, in the case shown in FIG. 5(B), unless the drive force applied to the magnetic head 15 by the retractor circuit is relatively large where the magnetic head 15 is above the outer peripheral portion of the magnetic disk 10, the magnetic head 15 cannot reach the landing zone 11. In contrast, in the case shown in FIG. 5(C), the magnetic head 15 will reach the landing zone 11 by a small drive force.
As stated above, when the magnetic head moves to the landing zone 11, there is not conducted a control such that a suitable retract current flows, resulting in incomplete retractive operation. This gives rise to a problem that the magnetic head stays on the recording surface. In such a case, there occurs a problem that the magnetic head damages the magnetic disk, with the result that data recorded on the magnetic disk is broken.